An organic light emitting phenomenon is an example of a conversion of current into visible rays by an internal process of a specific organic molecule. The organic light emitting phenomenon is based on the following principle. When an organic material layer is interposed between an anode and a cathode, if voltage is applied between two electrodes, electrons and holes are injected from the cathode and the anode to the organic material layer. The electrons and the holes injected into the organic material layer are recombined to form an exciton, and the exciton is fallen again to a bottom state to emit light. The organic light emitting device using the principle may be generally constituted by a cathode, an anode, and an organic material layer interposed therebetween, for example, an organic material layer comprising a hole injection layer, a hole transport layer, a light emitting layer, and an electron transport layer.
The materials used in the organic light emitting device are mostly pure organic materials or complexes of organic material and metal, and may be classified into a hole injection material, a hole transport material, a light emitting material, an electron transport material, an electron injection material, and the like according to its use. Herein, an organic material having a p-type property, that is, an organic material easily oxidized and electrochemically stable when the organic material is oxidized, is mainly used as the hole injection material or the hole transport material. Meanwhile, an organic material having an n-type property, that is, an organic material easily reduced and electrochemically stable when the organic material is reduced, is mainly used as the electron injection material or the electron transport material. A material having both p-type and n-type properties, that is, a material that is stable when the material is oxidized and reduced, is preferable as the light emitting layer material, and a material having high light emitting efficiency for conversion of the exciton into light when the exciton is formed is preferable.
In addition to the aforementioned matter, it is preferable that the material used in the organic light emitting device further have the following properties.
First, it is preferable that the material used in the organic light emitting device have excellent thermal stability. The reason is because joule heat is generated by movement of electric charges in the organic light emitting device. NPB, which has recently been used as the hole transport layer material, has a glass transition temperature of 100° C. or lower, and thus there is a problem in that NPB is difficult to be used in an organic light emitting device requiring a high current.
Second, holes or electrons injected into the organic light emitting device should be smoothly transported to the light emitting layer, and the injected holes and electrons should not be discharged to the outside of the light emitting layer in order to obtain a high efficiency organic light emitting device that can be driven at a low voltage. To this end, a material used in the organic light emitting device should have an appropriate band gap and HOMO or LUMO energy level. In the case of PEDOT:PSS currently used as a hole transport material in an organic light emitting device manufactured by a solution coating method, since a LUMO energy level thereof is lower than that of an organic material used as a light emitting layer material, it is difficult to manufacture an organic light emitting device having high efficiency and a long life span.
In addition, the material used in the organic light emitting device should have excellent chemical stability, electric charge mobility, and interfacial characteristic with an electrode or an adjacent layer. That is, the material used in the organic light emitting device should be little deformed by moisture or oxygen. Further, appropriate hole or electron mobility should be ensured so as to balance densities of the holes and of the electrons in the light emitting layer of the organic light emitting device, thus maximizing formation of excitons. Additionally, a preferable interface with an electrode including metal or metal oxides should be ensured for stability of the device.
Accordingly, there is a need to develop an organic material having the aforementioned requirements in the art.